U.S. patent application number 13/201093 was filed with the patent office on 2011-12-15 for mems device and process.
Invention is credited to Tsjerk Hans Hoekstra, Colin Robert Jenkins, Richard Ian Laming.
Application Number | 20110303994 13/201093 |
Document ID | / |
Family ID | 40548202 |
Filed Date | 2011-12-15 |
United States Patent
Application |
20110303994 |
Kind Code |
A1 |
Jenkins; Colin Robert ; et
al. |
December 15, 2011 |
MEMS DEVICE AND PROCESS
Abstract
A micro-electrical-mechanical system (MEMS) transducer comprises
a layer of dielectric material having an electrode formed in the
layer of dielectric material. A region of the layer of the
dielectric material is adapted to provide a leakage path which, in
use, removes unwanted charge from the layer of dielectric
material.
Inventors: |
Jenkins; Colin Robert;
(Livingston, GB) ; Hoekstra; Tsjerk Hans;
(Balerno, GB) ; Laming; Richard Ian; (Edinburgh,
GB) |
Family ID: |
40548202 |
Appl. No.: |
13/201093 |
Filed: |
February 12, 2010 |
PCT Filed: |
February 12, 2010 |
PCT NO: |
PCT/GB10/50241 |
371 Date: |
August 23, 2011 |
Current U.S.
Class: |
257/416 ;
257/415; 257/E21.158; 257/E29.324; 438/50 |
Current CPC
Class: |
H04R 19/04 20130101;
B81B 2203/04 20130101; B06B 1/0292 20130101; B81B 3/0008 20130101;
B81B 2201/0257 20130101; H04R 19/005 20130101 |
Class at
Publication: |
257/416 ;
257/415; 438/50; 257/E29.324; 257/E21.158 |
International
Class: |
H01L 29/84 20060101
H01L029/84; H01L 21/28 20060101 H01L021/28 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 13, 2009 |
GB |
0902480.3 |
Claims
1. A micro-electrical-mechanical system (MEMS) transducer
comprising: a layer of dielectric material; an electrode formed in
the layer of dielectric material, such that the layer of dielectric
material comprises a first dielectric layer and a second dielectric
layer provided on opposite sides of the electrode; wherein a region
of the layer of the dielectric material is adapted to provide a
leakage path which, in use, removes unwanted charge from the layer
of dielectric material.
2. A MEMS transducer as claimed in claim 1, wherein the region
comprises a portion of dielectric material that is provided between
a first side of the electrode and a corresponding outer surface of
the layer of dielectric material.
3. A MEMS transducer as claimed in claim 2, wherein the thickness
of the region is reduced compared to a second region, the second
region comprising a portion of dielectric material that is provided
between an opposite side of the electrode and a corresponding outer
surface of the layer of dielectric material.
4. A MEMS transducer as claimed in claim 1, wherein the region is
adapted such that, when biased during use, the region of dielectric
material operates in a tunneling mode of operation.
5. A MEMS transducer as claimed in claim 1, wherein the region is
adapted such that it comprises: a first impedance in a first plane,
the first plane comprising a plane that lies substantially axially
between a surface of the electrode and a surface of a second
electrode forming part of a capacitive transducer; and a second
impedance in a second plane, the second plane comprising a plane
that lies substantially orthogonal to the first plane; wherein the
first impedance is reduced compared to the second impedance.
6. A MEMS transducer as claimed in claim 1, wherein the composition
of the region is different to another region in the layer of
dielectric material.
7. A MEMS transducer as claimed in claim 1, wherein the region of
dielectric material has a different thickness to another region of
dielectric material in the same layer.
8. A MEMS transducer as claimed in claim 1, wherein the region of
dielectric material has a different composition to another region
of dielectric material in the same layer.
9. (canceled)
10. A MEMS transducer as claimed in claim 1, further comprising one
or more sound ports provided in the dielectric material, the sound
ports providing an opening between a first outer surface of the
dielectric layer and a second outer surface of the dielectric
material.
11. A MEMS transducer as claimed in claim 10, wherein at least one
sound port passes through the electrode.
12. A MEMS transducer as claimed in claim 11, wherein a part of the
electrode is exposed at an inner surface of a sound port, the
exposed part of the electrode, in use, acting to remove unwanted
charge.
13. A method of forming a micro-electrical-mechanical system (MEMS)
transducer, the method comprising the steps of: depositing a first
layer of dielectric material; depositing an electrode; depositing a
second layer of dielectric material, such that the first layer of
dielectric material and second layer of dielectric material are
provided on opposite sides of the electrode; and wherein the step
of depositing the first layer of dielectric material comprises
forming a leakage path which, in use, removes unwanted charge from
the first layer of dielectric material.
14. A method as claimed in claim 12, wherein the step of depositing
the first layer of dielectric material comprises the step of
depositing a layer of dielectric material having a reduced
thickness compared to the second layer of dielectric material.
15. A method as claimed in claim 13, wherein the step of depositing
the first layer comprises the step of deposited the first layer
such that, when biased during use, the first layer of dielectric
material operates in a tunneling mode of operation.
16. A method as claimed in claim 15, wherein the step of depositing
the first layer comprises the step of depositing the first layer
with a predetermined thickness such that it operates in a tunneling
mode of operation.
17. A method as claimed in claim 15, wherein the step of depositing
the first layer comprises selecting one or more deposition
parameters that form a first layer having a tunneling mode of
operation during use.
18. A method as claimed in claim 13, wherein the first layer
comprises: a first impedance in a first plane that lies
substantially orthogonal to the plane of the first layer; and a
second impedance in a second plane that lies substantially parallel
to the plane of the first layer; wherein the first impedance is
reduced compared to the second impedance.
19. A method as claimed in claim 13, wherein the composition of the
first layer is different to the composition of the second
layer.
20. A method as claimed in claim 13, wherein a region of the first
layer has a different thickness to another region of the first
layer.
21. A method as claimed in claim 13, wherein a region of the first
layer has a different composition to another region of the first
layer.
22. A method as claimed in claim 13, further comprising the step of
forming one or more sound ports that extend through the first and
second layers of dielectric material.
23. A method as claimed in claim 22, wherein the step of forming
one or more sound ports comprises forming at least one sound port
through the electrode.
24. A method as claimed in claim 23, wherein the step of forming a
sound port through the electrode comprises exposing a part of the
electrode, the exposed part of the electrode, in use, acting to
remove unwanted charge.
25. An electronic device comprising a micro-electrical-mechanical
system (MEMS) transducer as claimed in claim 1.
26. A communications device comprising a
micro-electrical-mechanical system (MEMS) transducer as claimed in
claim 1.
27. A portable telephone device comprising a
micro-electrical-mechanical system (MEMS) transducer as claimed in
claim 1.
28. An audio device comprising a micro-electrical-mechanical system
(MEMS) transducer as claimed in claim 1.
29. A computer device comprising a micro-electrical-mechanical
system (MEMS) transducer as claimed in claim 1.
30. A vehicle comprising a micro-electrical-mechanical system
(MEMS) transducer as claimed in claim 1.
31. A medical device comprising a micro-electrical-mechanical
system (MEMS) transducer as claimed in claim 1.
32. An industrial device comprising a micro-electrical-mechanical
system (MEMS) transducer as claimed in claim 1.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a MEMS device and process, and in
particular to a MEMS device and process relating to a transducer,
and in particular a capacitive transducer such as a MEMS capacitive
microphone.
BACKGROUND OF THE INVENTION
[0002] Consumer electronics devices are continually getting smaller
and, with advances in technology, are gaining ever increasing
performance and functionality. This is clearly evident in the
technology used in consumer electronic products such as, for
example, mobile phones, laptop computers, MP3 players and personal
digital assistants (PDAs). Requirements of the mobile phone
industry, for example, are driving components to become smaller
with higher functionality and reduced cost. For example, some
mobile phones now require multiple microphones for noise
cancelling, or accelerometers to allow inertial navigation, while
maintaining or reducing the small form factor and aiming at a
similar total cost to previous generation phones.
[0003] This has encouraged the emergence of miniature transducers.
For example, in respect to speech applications, initially electret
microphones were used to capture speech, but more recently
micro-electrical-mechanical (MEMS) transducers have been
introduced. MEMS transducers may be used in a variety of
applications including, but not limited to, pressure sensing,
ultrasonic scanning, acceleration monitoring and signal generation.
Traditionally such MEMS transducers are capacitive transducers some
of which comprise one or more membranes with electrodes for
read-out/drive deposited on the membranes and/or a substrate.
Relative movement of these electrodes modulates the capacitance
between them, which then has to be detected by associated
electronic circuitry such as sensitive electronic amplifiers.
[0004] FIG. 1 shows a schematic diagram of a capacitive microphone
device. The capacitive microphone device comprises a flexible
membrane 11 that is free to move in response to pressure
differences generated by sound waves. A first electrode 13 is
mechanically coupled to the flexible membrane 11, and together they
form a first capacitive plate of the capacitive microphone device.
A second electrode 15 is mechanically coupled to a generally rigid
structural layer or back-plate 17, which together form a second 2
capacitive plate of the capacitive microphone device. The membrane
11 and back-plate 17 are formed from a dielectric material, for
example silicon nitride.
[0005] The flexible membrane 11 is free to move in response to a
stimulus, for example sound or pressure waves. This movement of the
flexible membrane causes the capacitance between the first and
second electrodes 13, 15 to change. In use the capacitive
microphone device is typically arranged to drive into a high
impedance circuit, such that the charge on the capacitor does not
change significantly. Therefore, the change in the capacitance of
the transducer due to the stimulus results in a change 11V in the
voltage across the transducer capacitance. This change in voltage
can be detected and processed to provide an electrical output
signal.
[0006] FIG. 2 shows the membrane 11 and back-plate 17 of FIG. 1 in
greater detail, together with their respective electrodes 13 and
15. The first and second electrodes 13, are biased by a voltage
source 19, for example a charge pump providing a bias voltage of
12v.
[0007] As can be seen from FIG. 2, the second electrode 15 is
formed within the back-plate 17 such that it is surrounded by
dielectric material. The electrode is encapsulated within the layer
of dielectric material, i.e. the back-plate 17, for a number of
reasons.
[0008] Firstly, by forming the electrode15 within the back-plate 17
the dielectric material acts to protect the electrode 15 from
harmful environmental effects. For example, the dielectric material
prevents the metal electrode 15 from corroding or oxidising.
[0009] Secondly, by forming the electrode 15 within the back-plate
17 the fabrication process is improved. This is because it is
easier to deposit a metal electrode onto silicon nitride than it is
to deposit a metal electrode onto polyimide (polyimide being
typically used as a sacrificial layer during the fabrication
process for forming the air gap that exists between the back-plate
17 and the membrane 11).
[0010] Thirdly, encapsulating the electrode 15 within the
back-plate 17 has the effect of lowering the stiction forces that
exist when the membrane 11 is deflected towards the back-plate
17.
[0011] A disadvantage of the arrangement shown in FIG. 2 is that
the dielectric layer under the electrode 15 of the back-plate 17 is
susceptible to charging, especially for a dielectric layer
comprising silicon nitride. This is particularly the case when the
resistance of the air gap between the first and second electrodes
13, 15 is reduced, for example in a humid atmosphere. This causes
ions to drift through the air gap and accumulate onto the lower
surface of the back-plate 17, causing a charge accumulation on the
lower surface of the dielectric layer. As can be seen from FIG. 3,
a positive charge accumulates on the upper surface of the second
electrode 15, while a negative charge accumulates on the lower
surface of the first electrode 13.
[0012] The charge on the silicon nitride layer can build up to the
maximum of the applied voltage (e.g. 12V). This has the
disadvantage of reducing the sensitivity of the MEMS device.
[0013] In the example shown in FIG. 3 a 2v charge in the silicon
nitride layer (i.e. between the underside of the second electrode
15 and the lower surface of the back-plate 17) leads to a
sensitivity loss of:
20 log(10/12)=1.6 dB
[0014] Similarly, a 6V charge in the silicon nitride layer gives an
attenuation of 6 dB, and a 9V charge an attenuation of 12 dB.
[0015] Any charge accumulated in the silicon nitride layer has a
very long decay time, possibly days or weeks, which is obviously
much longer than the response time of the MEMS device.
[0016] FIG. 4 is a graph illustrating the sensitivity reduction on
two microphones after temperature humidity bias testing (at 85
C./85.degree. A).
SUMMARY OF THE INVENTION
[0017] It is an aim of the present invention to provide a MEMS
device, and a method of fabricating a MEMS device, that reduces or
removes the undesired effect mentioned above.
[0018] According to a further aspect of the invention, there is
provided a micro-electrical-mechanical system (MEMS) transducer
comprising: a layer of dielectric material; an electrode formed in
the layer of dielectric material; wherein a region of the layer of
the dielectric material is adapted to provide a leakage path which,
in use, removes unwanted charge from the layer of dielectric
material.
[0019] According to a first aspect of the invention, there is
provided a method of forming a micro-electrical-mechanical system
(MEMS) transducer, the method comprising the steps of: depositing a
first layer of dielectric material; depositing an electrode;
depositing a second layer of dielectric material; wherein the step
of depositing the first layer of dielectric material comprises
forming a leakage path which, in use, removes unwanted charge from
the first layer of dielectric material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] For a better understanding of the invention, and to show
more clearly how it may be carried into effect, reference will now
be made, by way of example only, to the accompanying drawings in
which:
[0021] FIG. 1 is schematic cross-sectional view of a MEMS
microphone according to the prior art;
[0022] FIG. 2 shows the back-plate, membrane and associated
electrodes of FIG. 1 in greater detail;
[0023] FIG. 3 illustrates the accumulation of charge in the device
of FIGS. 1 and 2;
[0024] FIG. 4 is a graph illustrating the sensitivity reduction on
two microphones after temperature humidity bias testing;
[0025] FIG. 5 shows a MEMS device according to an embodiment of the
invention;
[0026] FIG. 6 shows the electrical properties of silicon nitride,
and a Fowler-Nordheim plot;
[0027] FIG. 7 illustrates measured conductivities through 50 nm of
silicon nitride using different materials;
[0028] FIG. 8 illustrates the charge decay of silicon nitride
having helium depletion;
[0029] FIG. 9 illustrates the charge decay of silicon nitride
having silicon enrichment; and
[0030] FIG. 10 shows a further embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] The invention will be described in relation to a MEMS
transducer in the form of a MEMS capacitive microphone. It will be
appreciated, however, that the invention is equally applicable to
any form of capacitive transducer including, but not limited to
pressure sensors, ultrasonic transducers, acceleration monitoring
and signal generation transducers.
[0032] Furthermore, although the invention will be described in
relation to a back-plate of a MEMS device, it will be appreciated
that the invention is equally applicable to any other dielectric
layer of a MEMS device having an associated electrode, for example
a membrane having a layer of dielectric material between its
electrode and the other electrode of the capacitive transducer, for
example if the electrode 13 was provided on the underside of the
membrane 11, or encapsulated therein.
[0033] It is also noted that, although the embodiments of the
invention are described in relation to a back-plate formed from
silicon nitride, it will be appreciated that the invention is
applicable with other dielectric materials.
[0034] According to the invention a leakage path is provided in a
region of the back-plate for preventing or removing any unwanted
charge from accumulating in the back-plate.
[0035] Referring to FIG. 5, according to one embodiment the
thickness of the back-plate is reduced on the underside of the
electrode 15, Le. in a region 17a of the back-plate. It will be
appreciated that the term "underside" refers to the side of the
electrode which faces the other electrode of the capacitive
transducer, i.e. the side that faces electrode 13. Reducing the
thickness of the back-plate 17 in this region 17a has the effect of
lowering the impedance of the dielectric layer between the
electrodes 15, 13. In other words, the impedance of the dielectric
layer under the electrode 15 is reduced in a plane that lies along
an axis X-X, this axis X-X being an axis that runs in the general
direction from one electrode 15 to another electrode 13.
[0036] It is noted that the back-plate in a MEMS device such as
that shown in FIG. 5 may be formed by depositing a first layer of
silicon nitride, depositing the electrode 15 on a portion of the
first layer of silicon nitride, and depositing a second layer of
silicon nitride over the electrode 15 and first layer of silicon
nitride, thereby encapsulating the electrode with silicon nitride
to form the back-plate 17. The first layer of silicon nitride may
be deposited over a sacrificial layer (not shown), which is
deposited during the fabrication process in order to support the
formation of the back-plate 17, but then removed to provide the air
gap between the back-plate 17 and the membrane 11. Therefore, the
layer on the underside of the back-plate 17 can be reduced in
thickness by depositing the first layer with reduced thickness
compared to the second layer.
[0037] According to one embodiment, the thickness of the dielectric
layer in the region 17a is reduced such that the dielectric layer
behaves in a tunneling mode of operation, such that when the
capacitive transducer is biased during use any unwanted charge
stored in the region 17a of the back-plate is removed.
[0038] FIG. 6 shows a Fowler-Nordheim plot illustrating the
electrical properties of silicon nitride, including an ohmic region
and a tunneling region. The silicon nitride layer on the underside
of the electrode 15 is deposited such that the silicon nitride
layer is adapted to operate in the tunneling mode.
[0039] This enables a current to flow through the silicon nitride
layer which discharges any accumulated charge, thus making the air
gap the key insulator in the capacitive circuit.
[0040] It will be appreciated that reducing the thickness of the
back-plate on the underside of the electrode to a bare minimum
thickness can have an undesired effect of reducing the insulating
effect of the dielectric of the capacitive microphone device, which
could lead to device collapse and short circuit.
[0041] Therefore, according to another embodiment, one or more
parameters of the silicon nitride layer and/or one or more
parameters associated with the deposition of the silicon nitride
layer are altered to provide a region that has improved charge
removal properties for a given thickness of silicon nitride.
Preferably this is done in addition to reducing the thickness as
mentioned above. However, it is noted that the reduction in the
thickness of the silicon nitride can be altered in relation to what
other parameters are changed in the composition of the silicon
nitride, or its deposition, thus enabling the thickness to be
reduced by less than would otherwise be needed.
[0042] The parameters are selected such that the dielectric layer
underneath the electrode has a first resistance in a first
direction X-X (i.e. between the surface of the electrode and the
corresponding outer surface of the back-plate), and a second
resistance in a second direction Y-Y (Le. along the body of the
silicon nitride layer). A very high sheet resistance is maintained
in the nitride layer in direction Y-Y in order to prevent
undesirable leakage currents from the 12V charge pump input, (which
would otherwise cause noise at the microphone output).
[0043] A key parameter is controlling the layer thickness and
separation of the conductive features such that the nitride layer
appears to be resistive through its thickness, but is a near
perfect insulator over the sheet resistance.
[0044] FIG. 7 shows current conduction through 50 mm thick nitride
layers of different compositions. An ohmic conductive region can be
seen for all the materials where the current is greater than 1e-5A.
The material should be controlled so that a measure of conductivity
is always present through the sheet (for example, material 4C1 in
FIG. 7), but where the conductivity across the sheet is minimised
through aspect ratio control and material choice.
[0045] Silicon Nitrides with different deposition properties have
been measured for charge decay using microphone die with 300 nm
nitride below the back-plate electrode. Standard silicon nitride
has been found to have very long charge decay times extending to
weeks or months. Silicon nitrides deposited using helium depletion
have been seen to decay to 2.5V in about 100 hours, as shown in
FIG. 8. Silicon nitrides deposited which are 1.3% silicon rich have
seen charge decay to 2.5V in about 20 minutes, as shown in FIG.
9.
[0046] The electrodes 15, 13 are preferably placed in a central
region of the back-plate and membrane, i.e. rather than over the
entire surface of the membrane or back-plate, thus having the
advantage of maximising the leakage path via the sidewalls of the
device.
[0047] In the embodiments described above the sound ports are shown
passing through the dielectric material and the electrode. It will
be appreciated, however, that the invention is equally applicable
to a device where the sound ports do not pass through the electrode
itself.
[0048] In the embodiment of FIG. 5, i.e. having sound ports passing
through the electrode, it can be seen that the electrode is
encapsulated within the dielectric material. In other words, the
edges of the electrode 15 are not exposed on the inner surfaces of
the sound ports. This is achieved by etching holes having a first
diameter in the electrode 15 prior to the dielectric layer being
formed over the electrode 15, such that when the smaller diameter
sound ports are subsequently etched through the back-plate, the
electrode 15 is not exposed on any of the inner surfaces of the
sound ports.
[0049] However, according to another embodiment of the invention,
the electrode is not etched prior to forming the layer of
dielectric over the electrode. Thus, when the sound ports are
etched through the dielectric material and the electrode, the
electrode will be exposed on the inner surface of the sound ports.
Although this has the disadvantage of not providing an
environmental barrier, it has the advantage of providing a surface
leakage path 101 between the edge of the exposed electrode and the
lower surface of the back-plate.
[0050] According to another embodiment, the device can be
configured such that certain sound ports have the electrode
exposed, while other sound ports have the sound port encapsulated.
This has the advantage of enabling the effect of the leakage path
to be controlled more accurately. The variation in the number of
sound ports having the electrode exposed can be determined during
the step of etching holes in the electrode, i.e. prior to the top
layer of the back-plate being formed. For example, some holes in
the electrode can be configured to have diameters that are larger
than the sound ports (such that the electrode does not become
exposed when the sound ports are subsequently etched), while other
holes in the electrode can be etched to have diameters that match
the diameters of the sound ports (or not etched at all).
[0051] It is noted that the invention may be used in a number of
applications. These include, but are not limited to, consumer
applications, medical applications, industrial applications and
automotive applications. For example, typical consumer applications
include laptops, mobile phones, PDAs and personal computers.
Typical medical applications include hearing aids. Typical
industrial applications include active noise cancellation. Typical
automotive applications include hands-free sets, acoustic crash
sensors and active noise cancellation.
[0052] It should be noted that the above-mentioned embodiments
illustrate rather than limit the invention, and that those skilled
in the art will be able to design many alternative embodiments
without departing from the scope of the appended claims. The word
"comprising" does not exclude the presence of elements or steps
other than those listed in a claim, "a" or "an" does not exclude a
plurality, and a single feature or other unit may fulfil the
functions of several units recited in the claims. Any reference
signs in the claims shall not be construed so as to limit their
scope.
* * * * *